1. Field of the Invention
This invention relates generally to the field of geophysical prospecting and more particularly to an apparatus for the generation of seismic waves in the earth by impulsive means.
2. Description of the Prior Art
Injection of seismic signals into the earth by means of apparatus which utilizes compressed gas to propel an impact mass against a rigid earth contacting target is described in U.S. Pat. No. 4,284,164 to Tom P. Airhart and U.S. Pat. No. 4,284,165 to Tom P. Airhart, et al, both entitled "Acoustic Pulse Generator" and issued Aug. 18, 1981, the contents of such patents being hereby incorporated herein by reference. A significant advantage of such devices is their ability to produce rapidly repeated high energy seismic signals of carefully controlled energy content, and they are well suited to the production of either pure compressional waves or combined compressional and shear waves, devices of the latter capability being commonly termed "multi-component" sources.
In accordance with the teachings of such prior art and related patents, compressed gas is admitted on signal within the bore of a hollow cylindrical housing at high pressure, say 300 p.s.i., in a manner to drive a movable elongated piston from a cocked position within such bore so as to strike an earth-contacting target, positioned adjacent one end of the housing, after which the piston is returned to the cocked position for a subsequent shot. If the compressed gas is prevented from escaping from the housing during each shot in accordance with the teachings of allowed U.S. patent application Ser. No. 06/841,637 entitled "Vent Means For Closed Air System" filed in the name of Tom P. Airhart on Mar. 20, 1986, incorporated herein by this reference, the return of such piston means repressurizes the system to substantially the same driving pressure.
To facilitate seismic surveying along a selected line or path, apparatus as described is normally mounted on a transport vehicle. Operation of hydraulic lift cylinders secured between the vehicle chassis and the target transfers the vehicle weight from its tires to such target. Means are provided for adjusting the angle which the housing bore makes with the vertical. In this way the apparatus may be operated either as a P-Wave or multi-component source. In operating such apparatus, the downward acceleration of the piston against the target is necessarily accompanied by a corresponding upward recoil against its housing. This recoil is in turn transmitted to the vehicle itself, which must be sufficiently heavy and stable to absorb this shock without experiencing unacceptable levels of mechanical stress or upsetting movements. This places certain constraints on the design of the overall system.
Typically, a seismic source of the type described is mounted on an off-road, four-wheeled "buggy" which, together with associated air compressors, hydraulic equipment and the like may easily weigh upwards of 50,000 pounds in addition to the very significant weight of the housing, piston and target. It would be advantageous however, to employ a smaller, lighter transport vehicle to carry and operate the seismic source. One reason is to meet the size and weight limitations imposed on "highway legal" vehicles. Another is to enable the source to operate with greater mobility on rugged terrain. By reducing operating pressure and leaving other design parameters unchanged, one can match the source to such smaller scale vehicles without intolerable reaction forces. But this necessarily degrades signal strength, because the amplitude of the seismic signal produced is a function of the attained kinetic energy of the descending piston at the moment of target impact. This energy is in turn directly proportional to the operating pressure for a given piston mass and length of stroke.
Thus, it is one object of this invention to provide a vehicle-transported seismic source of the character described which minimizes vehicle size and weight requirements without sacrificing the strength and quality of the signal produced.
It is a more particular object of the invention to provide a seismic source of the character described wherein operating pressure is reduced without corresponding reduction in signal strength.
Other and further objects and advantages of this invention will become apparent from a consideration of the detailed description set forth below taken in conjunction with the accompanying drawings.